Secondary recovery of hydrocarbons is carried out in an oil field by injecting several injection wells with appropriate injection fluid. The injection fluid radiates away from the injection wells and flows toward several producing wells, carrying hydrocarbons therewith and thereby increasing the production of the oil field. Fresh water, salt water, CO.sub.2, N.sub.2, and flue gases are common injection fluids used to drive the hydrocarbon from the partially depleted formation towards the producing wells. The wells are arranged in different patterns for optimizing the secondary recovery efforts.
The rate of flow of the injection fluid into the downhole formation is governed by the structural integrity of the injection borehole. Should the pressure exerted by the injection fluid on the well bore exceed a maximum value, the piping of the wellbore and the adjacent formation may rupture, causing contamination of the overlying aquifer. This is a disastrous event and the laws are very strict to assure that the intrusion of harmful chemicals into an aquifer does not occur as a result of employing too great a pressure on the injection well.
The vast quantity of injection fluid flowing into the injection well ultimately results in plugging of the depleted hydrocarbon bearing formation due to all sorts of things, such as accumulation of unwanted material, chemical reactions, translocation of debris, and similar phenomena that increase the pressure drop across the formation and require that the flow rate of the injection material be progressively decreased so as to avoid exceeding the maximum permissible pressure on the wellbore by the injection fluid. Iron sulphide, calcium sulphide, various carbonates, and debris are some of the contaminants that cause plugging and they are difficult to remove because of the isolation thereof by hydrocarbon based inhibitors that are invariable employed for reducing corrosion of the pipe in the borehole. These hydrocarbon encapsulate the contaminants making removal with acidizing techniques difficult and greatly increases the quantity of acid required to react with the contaminants.
A secondary recovery operation in an injection field will therefore eventually encounter a few wells of a plurality of wells that are slowly dropping in volumetric flow until the flow rate drops to an undesirable value. Eventually it becomes desirable for these few poor wells to be rejuvenated, that is, somehow treated and made to accept the desired volumetric flow at less than the maximum allowable pressure. This requires removing at least part of the contaminating material causing the formation to be plugged to thereby reduce the flow restriction to a value that allows the borehole to accept a reasonable quantity of the injection fluid at an acceptable pressure effected on the wellhead, formation, and borehole.
In the past it has been found that the obstruction, or contaminant causing a plugged formation, can be removed by treatment with chlorine dioxide and acrolein, followed by acidizing. The use of these two chemical compounds is environmentally objectionable because they are deadly poisonous and there is the possibility that they will ultimately contaminate our water supply. There is likelihood that under the best of conditions these compounds ultimately will appear in the produced oil, and possibly much worse results can occur if anything malfunctions with the overall injection system.
It is therefore desirable that alternate methods of unplugging hydrocarbon bearing formations associated with injection fields undergoing secondary recovery be safely substituted for the present method of treatment, and such a solution to this problem is the subject of this invention.